An MSC pool is defined as a pool of MSC/VLR nodes linked to a number of RAN nodes. In the GSM standard RAN nodes are called BSC nodes and in the WCDMA standard RAN nodes are called RNC nodes. The invention is applicable to telecommunications systems such as GSM and WCDMA systems although the below description refers to GSM system only. Each BSC is connected to each of the MSC/VLR in the MSC/VLR pool.
In [1] the approach of having a pool of integrated core network nodes MSC/VLR has been selected and specified. The concept of a pooled core network has been proposed as a way to ease the scalability of the core network and to improve the service availability in the core network by limiting the effect of an outage of an MSC/VLR node.
When more capacity is needed, additional MSC/VLR nodes can be added to the pool without increase of the location update and inter-MSC handover traffic. An MS will be registered in the same MSC/VLR node as long as the mobile is located within the radio network related to the MSC/VLR pool.
Between the BSC and MSC/VLR nodes terrestrial transmission resources are needed in order to transport the traffic channels. A traffic channel can for example be used for a speech call or a circuit switched data call.
The radio network controlled by a BSC node will generate a specific amount of traffic in terms of speech calls etc. and in order to match this traffic a number of terrestrial transmission resources need to be configured between the BSC and the MSC nodes. The number of terrestrial transmission resources that are needed depends on a number of factors like the requirement on the blocking probability at call set-up (during busy hour), the estimated busy hour traffic etc.
A BSC node connected to a pooled core network will need a number of terrestrial transmission resources for each MSC in the MSC pool. Since it is the MSC node that controls the allocation of the terrestrial transmission resources there will be a terrestrial transmission resource pool for each BSC-MSC connection.
Remember that each BSC node is connected to each of the MSC/VLR nodes in the MSC/VLR pool. Also remember that when an MS registers its presence in the system, it will register its presence in one of the MSC/VLR nodes in the core network. If it is the first time the MS performs a registration in the particular MSC pool, an MSC/VLR node will be selected among the MSC/VLR nodes in the pool; in all other cases the registration will be performed towards the MSC/VLR in which the MS is currently registered.
Since it is the MSC node that allocates the terrestrial transmission resources during a call set-up it might be the case that the call set-up fails due to shortage of terrestrial resources between the BSC and a specific MSC. Then the terrestrial resource shortage is detected, the MSC node will terminate the call set-up and request the BSC node to release radio resources allocated during the early phase of the call set-up.
As the traffic load in the system increases the transmission facilities between the MSC pool and the BSC nodes will be more and more loaded and finally a point is reached when there are no or only a few transmission facilities available for calls. If under these circumstances an MS makes a call, the call will be rejected. This case is of course serious per se, but not that serious, since the possibility is low to find any non-occupied transmission at other locations in the down loaded system assuming a load balanced MSC pool.
Suppose the transmission facilities between an individual BSC node and the MSC pool have a major failure and go down. Also suppose the traffic load is low. If under these circumstances an MS makes a call, the call will be rejected since there are no transmission facilities available between the BSC node and the MSC that allocates the transmission resources. This case, below referred to as “local shortage problem”, is serious, since there are transmission facilities available at other locations in the system. Such non-occupied transmission facilities will exist between the BSC node and other pooled MSCs than the one at which the MS is currently registered.
Given a predefined traffic load it can be shown that if MSC nodes are pooled in the manner described above, the amount of transmission facilities required to cope with said traffic load will increase compared to the case when the MSC nodes are not pooled. Pooling per se will thus increase the need of transmission facilities.